1. Field of the Invention
This invention relates to photon detection in general, and more particularly to amplifiers for single photon readout of semiconductor photodetectors in pixellated imaging arrays.
2. Description of the Related Art
Optical sensors transform incident radiant signals in the X-ray (xcex less than 0.001 xcexcm), ultraviolet (xcex=0.001-0.4 xcexcm), visible (xcex=0.4-0.8 xcexcm), near infrared (IR) (xcex=0.8-2 xcexcm), shortwave IR (xcex=2.0-2.5 xcexcm), mid IR (xcex=2.5-5 xcexcm), and long IR (xcex=5-20 xcexcm) bands into electrical signals that are used for data collection, processing, storage and display such as real-time video. Available conventional photodetectors such as photodiodes and photoconductors are inexpensive, exhibit bandwidths that support current video frame rates, are sensitive to wavelengths well into the long IR band, and exhibit a high degree of uniformity from pixel to pixel when used in an imaging array. However, these photodetectors have no gain, i.e. each incident photon generates, at most, a single electron; these imaging systems thus work very well only in moderate to bright light conditions. They provide electrical signals at low light levels that are too small to be read-out by conventional readout circuits.
In conditions of low ambient light, the standard photodetector is often replaced with an avalanche photodiode that provides gain such that conventional readout circuits, such as charge coupled devices, i.e. CCDs, can read out the data at video frame rates with a high signal-to-noise ratio (SNR). The fabrication of avalanche photodiodes is much more difficult and expensive than standard photodetectors because they must exhibit very high controlled gain and very low noise. Furthermore, currently available avalanche photodiodes exhibit relatively poor uniformity, are constrained to shorter wavelengths than standard photodetectors (0.7 xcexcm), and have limited sensitivity due to their relatively low quantum efficiency. Imaging intensified systems use an array of avalanche photodiodes or microchannel plates to drive respective display elements such as CCDs or phosphors, and have even lower wavelength capabilities (approximately 0.6 xcexcm max) due to the limitations of the photodiode.
Chamberlain et al. xe2x80x9cA Novel Wide Dynamic Range Silicon photodetector and Linear Imaging Arrayxe2x80x9d IEEE Transactions on Electron Devices, Vol. ED-31, No. 2, February 1984, pp. 175-182 describes a gate modulation technique for single photon readout of standard photodetectors. Chamberlain provides a high gain current mirror that includes a load FET whose gate is connected to its drain to ensure subthreshold operation and to eliminate threshold voltage (VT) non-uniformity. The pixel-to-pixel VT non-uniformity associated with standard silicon CMOS fabrication processes would otherwise substantially degrade the performance of the imaging array. The signal from the photodetector is injected into the load FET thereby producing a signal voltage at the gate of a gain FET. This signal modulates the gain FET""s gate voltage, thereby storing integrated charge in a storage capacitor that is read out and reset via a pair of FET switches.
Although Chamberlain""s particular gain modulation technique provides a large dynamic range and is useful for detecting signals across a broad spectral range, the current mirror""s bandwidth severely restricts the imaging array""s bandwidth. Specifically, the dominant RC time constant is the parallel combination of the photodetector""s capacitance and the resistance of the load FET. In subthreshold operation, the FET""s transconductance is very low and, hence, its load resistance is very large, at  greater than 1014 ohms; the minimum resulting RC time constant is on the order of seconds. Thus, Chamberlain""s gate modulation technique is only practically useful for imaging daylight scenes or static low-light-level scenes such as stars. Furthermore, to achieve large current gain, the load FET is typically quite small. As a result, the load FET exhibits substantial 1/f noise, which under low light conditions seriously degrades the performance of the imaging array.
Kozlowski et al. xe2x80x9cSWIR staring FPA Performance at Room Temperature,xe2x80x9d SPIE Vol. 2746, pp. 93-100, April 1996 describes a phenomenon called xe2x80x9cnight glowxe2x80x9d in the short wavelength infrared (SWIR) band that enables detection on very dark nights where photon flux is on the order of one hundred photons per imaging frame. Kozlowski details InGaAs and HgCdTe detector arrays for use with two different readout circuits. Both use current mirrors similar to Chamberlain, but one also buffers the detector node to maintain constant detector bias. Unlike SWIR band and longer wavelength detector arrays, near IR and visible detectors are not sensitive to changes in detector bias, and thus buffering to maintain constant bias is irrelevant. More importantly, the buffering enhances the circuit bandwidth such that the bandwidth is significantly enhanced; yet the bandwidth is still insufficient for displaying video at very high frame rates. The negative feedback amplifier A1, in U.S. Pat. No. 5,929,434, reduces the input impedance of the high-gain circuit and thereby enhances its bandwidth. In the case where the buffer amplifier is approximated to have infinite voltage gain and finite transconductance, the dominant pole is given by:       τ          B      -      L        =            C      f              g              m        Q1            
where Cf is the effective feedback capacitance of the buffer amplifier from its output to its input. Assuming a cascoded amplifier configuration, the gate-source capacitance of Q1 is dominant and Cf is set by the gate-to-source capacitance of the subthreshold FET Q1. This is approximately given by the parasitic metal overlap capacitance. Assuming a minimum width transistor in 0.25 xcexcm CMOS technology, for example, the minimum Cf will be about 0.1 fF. Though this facilitates single photon sensing at video frame rates, additional boost is needed to support imaging at high frame rates well above 30 to 60 Hz.
Merrill finally teaches in U.S. Pat. No. 6,069,376 a pixel amplifier (FIG. 6) with speed switch suitable for still camera applications. This apparatus provides high-bandwidth signal integration with downstream gain, but its sensitivity is limited by the generation of reset noise at the storage element. Furthermore, a method is not provided for maximizing the signal""s dynamic range at the input to the amplifier.
The invention is a photodetector readout circuit, with extremely high sensitivity, capable of single-photon detection. A photodetector (preferably a photodiode) integrates a small-signal photocharge on the detector capacitance in response to incident photons, producing a photodetector output signal. A buffer amplifier is arranged to receive the photodetector output signal and to produce a buffered photodetector output signal. A coupling capacitor, has a first terminal connected to the buffered output signal and a second terminal connected to a signal input of a signal amplifier. The coupling capacitor shifts signal level at the input to the signal amplifier by an offset voltage. An electronic offset reset switch, connected to the coupling capacitor, allows resetting of the offset voltage, preferably during a simultaneous reset of the photodiode. When sampling of the photodiode signal begins, the offset across the coupling capacitor is also clamped. This effects correlated double sampling of the photogenerated signal, and facilitates elimination of correlated noise generated by resetting (discharging) photodetector capacitance. An adjustment voltage is also preferably summed with signal at the input of the signal amplifier, to set the operating point of the signal amplifier above threshold and thereby improve transimpedance, dynamic range, and linearity.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: